1. Field of the Invention
The invention relates to a hearing aid, wherein a permanent magnet, which is fixed at the patient's incus and which is driven by a coil, is used for stimulating the patient's hearing by causing the ossicular chain to vibrate according to the output signals from the hearing aid signal processor.
2. Description of Related Art
An example of such type of hearing aid is described in U.S. Pat. No. 5,558,618, wherein the hearing aid comprises an external unit comprising a microphone, an audio signal amplifier and a radio frequency (RF) transmitter and an implantable unit comprising a RF receiver, a coil driver circuitry and the coil which drives the magnet. The coil is fixed at a casing which houses the electronic circuitry of the internal unit. The casing is mounted at a retaining member fixed at the patient's skull within an opening drilled into the temporal bone. The position of the casing—and hence the position of the coil—is adjusted by a spindle drive. Once the retaining member has been fixed and the magnet has been fixed to the incus, the spindle drive is rotated until the coil rests against the magnet. Then the threaded shaft of the spindle drive is rotated a predetermined number of turns corresponding to a one millimeter movement of the casing, thereby adjusting the gap, i.e. the distance, between the coil and the magnet. It is mentioned in U.S. Pat. No. 5,558,618 that alternatively the gap could be physically measured; however, no information is given how such alternative embodiment could be realized.
U.S. Pat. No. 4,957,478 relates to a partially implantable hearing aid comprising a permanent magnet fixed at the ossicular chain and an external unit comprising a microphone, an audio signal processing unit and a coil for driving the magnet, which external unit is to be inserted into the ear canal of the patient. It is suggested that the patient should adjust the distance between the magnet and the coil according to the perceived hearing improvement by manually moving the external unit within the ear canal.